1. Field of the Invention
The present invention relates to novel oligomers and to their method of preparation.
2. Brief Description of the Prior Art
U.S. Pat. Nos. 4,208,309 and 4,113,702 disclose coating compositions comprising a mixture of a hydroxyl-containing polyester and a polyisocyanate curing agent. The polyester is formed from reacting a polyol, a polycarboxylic acid or anhydride and a monoepoxide. There is substantially no reaction between the polyisocyanate and the polyester until the mixture is applied to a substrate and cured.
There are a number of problems associated with the above-mentioned prior art. In the formation of the hydroxyl-containing polyester, if the choice of reactants and the reaction conditions are not carefully conntrolled, there can be a considerable amount of polyesterification and unreacted polyol formed. Unreacted polyol results in volatile organic content in the resultant coating composition which is undesirable because of increasingly stringent governmental emission standards. Another problem is the low viscosity of the polyester. Low viscosity is, of course, desirable for the formation of high solids coating compositions. However, if the coating compositions are spray applied, there will be an overspray which misses the substrate and accumulates in the painting booth. The low viscosity uncured resinous materials do not harden with time and can become very tacky and slippery creating a hazard to the personnel working in the area.
The present invention overcomes the aforementioned problems by providing a composition which results in substantially dry overspray and low volatile contents. In the present invention, a polyol is reacted with a 1,2-dicarboxylic anhydride under reaction conditions sufficient to form the half-ester with substantially no polyesterification product and unreacted polyol. This product is then reacted with an organic isocyanate such as a polyisocyanate. Reaction with a polyisocyanate results in chain extension and molecular weight buildup which is sufficient to overcome the tacky overspray problem mentioned above. This reaction product which contains carboxylic acid groups is then reacted with an epoxide to form a hydroxyl group-containing oligomer.